<!DOCTYPE html>
<html lang="zh" dir="ltr" class="client-nojs">
<head>
<title>explicit 说明符</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">
<link rel="shortcut icon" href="../../../common/favicon.ico">
<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-css:7:15cea3ec788a65b5187d4018eed543bf */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/explicit","wgTitle":"cpp/language/explicit","wgCurRevisionId":64399,"wgArticleId":2711,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":[],"wgBreakFrames":false,"wgPageContentLanguage":"zh","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"zh","wgMonthNames":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgMonthNamesShort":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgRelevantPageName":"cpp/language/explicit","wgUserVariant":"zh","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"zh","language":"zh","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-MathJax":1,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-js:7:258d7cd6aa9aa67dee25e01fb6a9e505 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style><!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]--></head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_explicit skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        <!-- /header -->
        <!-- content -->
<div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
<h1 id="firstHeading" class="firstHeading">explicit 说明符</h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">来自cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.html" title="cpp/language">language</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr"><div class="t-navbar" style=""><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.html" title="cpp/language">语言</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header">标准库头文件</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../freestanding.html" title="cpp/freestanding"> 自立与有宿主实现</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../named_req.html" title="cpp/named req">具名要求</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html#.E8.AF.AD.E8.A8.80.E6.94.AF.E6.8C.81" title="cpp/utility">语言支持库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concepts.html" title="cpp/concepts">概念库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../error.html" title="cpp/error">诊断库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility">工具库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string">字符串库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="https://zh.cppreference.com/w/cpp/container" title="cpp/container">容器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator">迭代器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../ranges.html" title="cpp/ranges"> 范围库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm">算法库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric">数值库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io">输入/输出库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale">本地化库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex">正则表达式库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic">原子操作库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread">线程支持库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../filesystem.html" title="cpp/filesystem">文件系统库</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental">技术规范</a> </td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../language.html" title="cpp/language">C++ 语言</a></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="classes.html" title="cpp/language/classes">类</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5">概述</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes">综述</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"><code>class</code>/<code>struct</code> 类型</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"><code>union</code> 类型</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="injected-class-name.html" title="cpp/language/injected-class-name">注入类名</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5">成员</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html" title="cpp/language/data members">数据成员</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">静态成员</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"><code>this</code> 指针</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested types">嵌套类</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">成员模板</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bit_field.html" title="cpp/language/bit field">位域</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"><code>using</code> 声明</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">成员函数</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">成员访问指定符</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constructor.html" title="cpp/language/initializer list">构造函数与成员初始化器列表</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96" title="cpp/language/data members">默认成员初始化器</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> 指定符</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"><code>explicit</code> 指定符</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">转换构造函数</a></td></tr>
<tr class="t-nv-h2"><td colspan="5">特殊成员函数</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">默认构造函数</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">复制构造函数</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">移动构造函数</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_assignment.html" title="cpp/language/copy assignment">复制赋值运算符</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_assignment.html" title="cpp/language/move assignment">移动赋值运算符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">析构函数</a></td></tr>
<tr class="t-nv-h2"><td colspan="5">继承</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">基与派生类</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ebo.html" title="cpp/language/ebo">空基类优化</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">虚成员函数</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">纯虚函数与抽象类</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"><tt>override</tt></a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"><tt>final</tt></a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>explicit</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>explicit (</b></code> <span class="t-spar">表达式</span> <code><b>) </b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">表达式</span>
</td>
<td> -
</td>
<td>  <a href="constant_expression.html#.E7.BB.8F.E8.BD.AC.E6.8D.A2.E7.9A.84.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/constant expression">经按语境转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 类型的常量表达式</a>
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> 指定构造函数<span class="t-rev-inl t-since-cxx11"><span>或转换函数</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>或<a href="class_template_argument_deduction.html" title="cpp/language/class template argument deduction">推导指引</a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>为显式，即它不能用于<a href="implicit_cast.html" title="cpp/language/implicit conversion">隐式转换</a>和<a href="copy_initialization.html" title="cpp/language/copy initialization">复制初始化</a>。</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 说明符可以与常量表达式一同使用。当且仅当该常量表达式求值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 时函数为显式。</div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<p>explicit 说明符只可出现于在类定义之内的构造函数<span class="t-rev-inl t-since-cxx11"><span>或转换函数</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>的 <span class="t-spar">声明说明符序列</span> 中。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>声明时不带函数说明符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 的<span class="t-rev-inl t-until-cxx11"><span>拥有单个无默认值形参的</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span>构造函数被称作<a href="converting_constructor.html" title="cpp/language/converting constructor">转换构造函数</a>。
</p><p>构造函数（除了<a href="copy_constructor.html" title="cpp/language/copy constructor">复制</a>/<a href="move_constructor.html" title="cpp/language/move constructor">移动</a>）和用户定义转换函数都可以是函数模板；<code>explicit</code> 的含义不变。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<p>跟随 <code>explicit</code> 的 <code>(</code> 记号被剖析成 explicit 指定符的一部分：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw1">explicit</span> <span class="br0">(</span>S<span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// C++20 中错误，C++17 中 OK</span>
    <span class="kw1">explicit</span> <span class="br0">(</span>operator <span class="kw4">int</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20 中错误，C++17 中 OK</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A
<span class="br0">{</span>
    A<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>      <span class="co1">// 转换构造函数</span>
    A<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span> <span class="co1">// 转换构造函数 (C++11)</span>
    operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B
<span class="br0">{</span>
    <span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
    <span class="kw1">explicit</span> B<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
    <span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    A a1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>      <span class="co1">// OK：复制初始化选择 A::A(int)</span>
    A a2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// OK：直接初始化选择 A::A(int)</span>
    A a3 <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>   <span class="co1">// OK：直接列表初始化选择 A::A(int, int)</span>
    A a4 <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：复制列表初始化选择 A::A(int, int)</span>
    A a5 <span class="sy1">=</span> <span class="br0">(</span>A<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span>   <span class="co1">// OK：显式转型进行 static_cast</span>
    <span class="kw1">if</span> <span class="br0">(</span>a1<span class="br0">)</span> <span class="sy4">;</span>      <span class="co1">// OK：A::operator bool()</span>
    <span class="kw4">bool</span> na1 <span class="sy1">=</span> a1<span class="sy4">;</span> <span class="co1">// OK：复制初始化选择 A::operator bool()</span>
    <span class="kw4">bool</span> na2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">bool</span><span class="sy1">&gt;</span><span class="br0">(</span>a1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：static_cast 进行直接初始化</span>
 
<span class="co1">//  B b1 = 1;      // 错误：复制初始化不考虑 B::B(int)</span>
    B b2<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>       <span class="co1">// OK：直接初始化选择 B::B(int)</span>
    B b3 <span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>   <span class="co1">// OK：直接列表初始化选择 B::B(int, int)</span>
<span class="co1">//  B b4 = {4, 5}; // 错误：复制列表初始化不考虑 B::B(int,int)</span>
    B b5 <span class="sy1">=</span> <span class="br0">(</span>B<span class="br0">)</span><span class="nu0">1</span><span class="sy4">;</span>   <span class="co1">// OK：显式转型进行 static_cast</span>
    <span class="kw1">if</span> <span class="br0">(</span>b2<span class="br0">)</span> <span class="sy4">;</span>      <span class="co1">// OK：B::operator bool()</span>
<span class="co1">//  bool nb1 = b2; // 错误：复制初始化不考虑 B::operator bool()</span>
    <span class="kw4">bool</span> nb2 <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1">&lt;</span><span class="kw4">bool</span><span class="sy1">&gt;</span><span class="br0">(</span>b2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：static_cast 进行直接初始化</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="converting_constructor.html" title="cpp/language/converting constructor"> 转换构造函数</a>
</li><li> <a href="initialization.html" title="cpp/language/initialization"> 初始化</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization"> 复制初始化</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization"> 直接初始化</a>
</li></ul>

<!-- 
NewPP limit report
Preprocessor visited node count: 1493/1000000
Preprocessor generated node count: 4615/1000000
Post‐expand include size: 46082/2097152 bytes
Template argument size: 10673/2097152 bytes
Highest expansion depth: 14/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_zh_:pcache:idhash:2711-0!*!0!!*!*!zh!* and timestamp 20200205072056 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    来自“<a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/language/explicit&amp;oldid=64399">https://zh.cppreference.com/mwiki/index.php?title=cpp/language/explicit&amp;oldid=64399</a>”                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        <div id="cpp-footer-base" class="noprint">
            <div id="footer">
                        <div id="cpp-navigation">
            <h5>导航</h5>
            <ul><li><a href="https://zh.cppreference.com/w/cpp/language/explicit">Online version</a></li><li>Offline version retrieved 2020-03-14 18:00.</li></ul></div>
                        <ul id="footer-info">
                                    <li id="footer-info-lastmod"> 本页面最后修改于2020年2月4日 (星期二) 23:20。</li>
                                    <li id="footer-info-viewcount">此页面已被浏览过21,806次。</li>
                            </ul>
                    </div>
        </div>
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.MathJax","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.074 secs. -->
	</body>
<!-- Cached 20200205162221 -->
</html>